why do people conduct an experiment using pH? whats the purpose of the pH scale? why do we use it?
You have not said what grade you are.
I'll answer the first part last.
pH scale is simple numerical means of assessing how acidic a solution might be. The concentration of acid (more correctly the hydrogen ion concentration) can cover a range from 0.1 mole per litre down to 0.00000000000001 moles per litre, which a huge range and very difficult to visualise. Thus there is a scale from 1 to 14 which covers this range, the pH scale. It turns out that neutral is pH 7, acid solutions have number less than 7 and alkaline numbers greater than 7.
Many reactions, especially biological reactions involving enzymes, are very sensitive to the acidity of the solution, i.e. the pH. Slight changes in the pH can make the reaction go faster or slower or not at all. Indeed low or high pH values can kill cells and denature (=unravel) enzymes. Thus it is important to control the pH of a reaction or the medium in which you have a biological study.
Does this help?